1. Class 6.2
Acids and Bases
Friday, October 8
CHEM 462
T. Hughbanks
Table 8.3 From Jolly, “Modern Inorganic Chemistry” Aqueous
pKa values of the binary hydrides of the nonmetals
CH4 NH3 H2O HF
~ 44 39 15.74 3.15
SiH4 PH3 H2S HCl
~ 35 27 6.89 -6.3
GeH4 AsH3 H2Se HBr
25 23 3.7 -8.7
H2Te HI
2.6 -9.3
Acidic and Basic Oxides
The oxides that one uses to form acids
and bases in aqueous solution often
have reactivity that reflects their acidic
or basic character.
Examples: Li2O, CaO, and BaO react
with water to form basic solutions and
can react with acids directly to form
salts. Likewise, SO3, CO2, and N2 O5
form acidic aqueous solutions and can
react directly with bases to give salts.

2. Oxides as Acid and Basic
Anhydrides
Basic Oxides (usually “ionic”)
CaO + 2H2O ––> Ca2+ + 2OH–,
moderately strong base
[O2–] + H2O ––> 2OH– K > 1022
Alkali metal and alkaline earth oxides are
basic (dissolve in acid).
Acidic Oxides (Acid Anhydrides)
element-oxygen (E–O) bond not broken on
dissolution
either
an E – O – E group is hydrolyzed by water
or
water is added across a double bond
Acidic Oxides not soluble in water will dissolve in basic
aqueous solutions to produce salts
e.g. As2O3 + 2NaOH(aq) ––> 2NaH2AsO3
(Often seen for anhydrides of weaker acids.)
Examples
O O O
N N + H2O 2 N H
O O O O O
O
P
O O O dissociation
O + 6 H2 O 4
P P
O O O HO P OH
P HO
O O
O
O
O C O + H2O C H
O O
H

3. Amphoteric Oxides
Dissolve in acids or bases - if strong enough.
E.g., BeO, SnO, certain forms of Al2O3
In strong acids: ZnO + 2HCl(aq) ––> ZnCl2(aq)
ZnO + 2HNO3(aq) ––> Zn(OH2)62+ + NO3-
In strong base: ZnO + 2NaOH(aq) ––>
2Na+(aq) + [Zn(OH4)]2– (aq)
Lux-Flood Concept (Oxide Solids)
Acid: Oxide ion acceptor
Base: Oxide ion donor
– A generalization that includes reactions
between solids when water never gets
involved. E.g.,
CaO + SiO2 CaSiO3
3 Na2 O + P2O5 2 Na3 PO4
NaOH + CO2 NaHCO3
Other Oxides
Many oxides (particularly of the transition
metals) are difficult to classify as acidic or
basic because redox chemistry is more
important.
e.g. MnO2 + 4HI (aq. conc.)
Mn2+(aq) + I2 + 2H2O + 2 I–

4. Hydrolysis of Metal Complexes
can give acidic solutions
pKa’s for [M(H2O)6]n+
[Fe(H2O)6]3+ 2.46
[Cr(H2O)6]3+ 3.89
[Al(H2O)6]3+ 4.85
[Fe(H2O)6]2+ 5.89
[Cu(H2O)6]3+ 7.49
[Ni(H2O)6]2+ 9.03
Examples of Acids From
Solvolyzed Metals
Aqua Acids (solvolysis) Al3+ solutions are acidic
AlCl3(s) + H2O Al3+(aq) + Cl–(aq)
(w/sm. amts of water, HCl gas is evolved):
AlCl3 + H2O “Al(OH)3” + 3HCl
AlCl2(OH)·nH2O complex
AlCl(OH)2 + mH2O
Highly charged cations with small
radii make for stronger acids:
[Fe(OH2)6]2+ fairly weak, [Fe(OH2)6]3+ is much
stronger. r(Fe3+) < r(Fe2+), the smaller, more
highly charged (more polarizing) cation
withdraws more e– density from coordinated
water.
More than size is involved:
r(Al3+) < r(Fe3+) ionic radii, but [Fe(OH2)6]3+
is stronger than [Al(OH2)6]3+
FeIII–O bonding probably more covalent
(smaller electronegativity diff. than Al/O).

5. Acid-Base behavior of Metal Ions not
so simple…
Ions like Al3+ and Fe3+ are often used as
examples of cations that form acidic
aqueous solutions, however acid-base
equilibria for the “ions” are not simple.
[Compare: ClCH2COOH pKa = 2.85
— has a “normal” titration curve (pH vs.
added base). What does it look like?.]
An acidic [Al(OH2)6]3+ solution will not titrate
this way. Why?
[Al(OH2)6]3+ is deprotonated, then…
e.g., [Al(OH2)6]3+ + CH3COO–
[Al(OH2)5(OH)]2+ + CH3COOH
condensation occurs:
H2 O 2+
H2 O OH2
2 Al 2 H2 O +
H2 O O
OH2 H
4+
H2 O H2 O
H
OH2
[OH– ]
H2 O O
H2 O
Al
O
Al
OH2
“Al(OH)3”
H
OH2 OH2
Transition Metals in High
Ox. States: Acidic
Metals in very high oxidation states form strong,
largely covalent, bonds with oxygen
––> weakens O-H bonds!
e.g. CrO42– weak conjugate base of chromic acid
2–
O O
+ H3 O+ O Cr O Cr O
dichromate
O O
e.g. MnO4– very weak conjugate base of
permanganic acid (both are powerful oxidants)

6. Lewis acids and Bases (orbital
viewpoint)
(1) Metal cations (Lewis acids) & Ligands
(Lewis bases)
(2) p-block Lewis Acids: Incomplete octets
e.g. B(CH3)3, AlCl3
• Review of bonding
• Lewis acidity Trend for BX3 (X = F, Cl, Br)?
AlCl3: exists as Al2Cl6 in gas phase
Familiar application: Friedel-Crafts
acylation
To be discussed later: Chloroaluminate
based Ionic Liquids
Molecules with low-lying LUMOs
E.g. (review CO2 and SO3 examples).
SO2: can function as a Lewis acid or
Lewis base:
SO2 as an acid (involvement of
LUMO)
SO2 as a Lewis base - use of S-
localized (HOMO) or O-localized lone-
pairs

7. SO3 is very electrophilic –– strong
Lewis acidity:
electronic structure (review)
Oleum: formed by adding SO3 to
H2SO4 .